The Structure of Merging Turbulent Jets Beneath a Small Quadrotor
2026-08-31 • Robotics
Robotics
AI summaryⓘ
The authors studied the airflow created below a small hovering quadrotor drone called Crazyflie 2.1. They used detailed imaging to see how the four rotor air streams combined into one jet as you go further down from the rotors. They found that after a certain distance, the airflow behaves similarly to well-known types of round jets, even though the drone is small and operates in a lower-speed regime. They also observed that some turbulence patterns still reflected the original four separate rotors. This helps better understand the airflow around small quadrotors, which is important for their performance and safe flying in groups.
quadrotorhoveringdownwash wakeparticle image velocimetry (PIV)turbulent statisticslow Reynolds numberjet scalingcenterline velocityrotor jetsself-similar flow
Authors
Anoop Kiran, Nora Ayanian, Kenneth Breuer
Abstract
The downwash wake of a hovering quadrotor governs both the vehicle's own performance and the safe spacing of multi-rotor formations. Prior measurements have largely characterized the mean flow, using single-point anemometry, volumetric tracking, or planar cuts through part of the rotor system. Higher-order turbulent statistics of the merged wake, and how they relate to canonical jet scaling, have remained unresolved, particularly for small quadrotors at the low-Reynolds-number end of the size range. Here, we present a detailed particle image velocimetry (PIV) study of the downwash of a hovering Crazyflie 2.1 quadrotor (arm length, $l = 46$ mm), sampled along a diagonal cut, passing through rotors along the symmetry axis of the quadrotor, and a front-rotor cut, passing through adjacent rotors. The four rotor jets merge into a single column by $z/l \approx 5$, beyond which the mean velocity profiles progressively approach the canonical round-jet self-similar form, collapsing by $z/l \approx 13$ when scaled by the local centerline velocity and half-width. Centerline decay and half-width growth follow canonical scaling laws with an effective source diameter $D_\text{eff} = 2.29\,l$, effective Reynolds number $Re_{D_\text{eff}} = 3 \times 10^4$, at the low end of the range over which canonical jet scaling has been established, and spreading and decay constants nonetheless within the canonical round-jet range. Resolving both cuts shows that the turbulent normal stresses retain a bimodal, cut-dependent signature of the four-rotor source throughout the measurement domain.